Hither to, as the prosthetic method in a deficit tooth portion, a method for using a bridge and a method for using a denture base have been generally employed.
However, since the method for using a bridge is a method in which healthy natural teeth in the both sides of the deficit tooth portion are cut to prepare an abutment tooth, with which is then engaged a metallic body provided in a dental prosthesis positioned in the deficit tooth portion, it had such defects that not only the healthy natural teeth must be cut to provide an engaging portion, but also since an occlusion pressure is not directly applied to the dental prosthesis part positioned in the deficit tooth portion, a bone loss is likely caused in the instant site.
Also, the method for using a denture base is a method in which a denture is fixed to a denture base made of a synthetic resin or the like to prepare a dental prosthesis. According to this method, the occlusal force which acts to the dental prosthesis is burdened by the remaining natural teeth and/or the oral mucosa, and therefore, this method involved such defects that a sense of incongruity is generated during the use of the dental prosthesis and that the matter wherein the dental base covers receptors of the taste lying scattered in the oral mucosa tissue, thereby generating blunting of the taste. In addition, it involved a serious defect that a loss of the residual ridge is caused during the use for a long period of time.
Thus, as a therapeutic method for overcoming these defects, a technology of dental implant in which an implant fixture acting as a retention balancer of a dental prosthesis is implanted in an implantation hole formed in a mandible of the deficit tooth portion and made act for the functions of a tooth root in a natural tooth, and a fixture is connected and fixed to the implant fixture in the oral cavity side thereof, whereby a dental prosthesis is fixed to this fixture of a dental prosthesis was developed and has been being exploited.
When a therapeutic method for using such a dental implant is exploited, the dental prosthesis can be fixed without covering the oral mucosa. Therefore, this method has such advantages that neither a sense of incongruity nor blunting of the taste is generated at the time when the dental prosthesis is installed, a use feeling like natural teeth is obtained, and that since a proper occlusal force is imparted to the mandible, the bone loss to be highly expected in case where no implant fixture is implanted can be suppressed at the minimum. Thus, this therapeutic method has rapidly developed to such an extent that it can be applied to not only the case where a single tooth is lost, or two or more teeth are locally lost but also the case of an edentulous jaw (i.e., all teeth are lost).
According to the therapeutic method for using such a dental implant, in order that an implantation hole in which an implant fixture is implanted is formed in a mandible of the deficit tooth portion and that after the implant fixture is implanted in this implantation hole, a screw hole exposing in the center in the oral cavity side of the implanted implant fixture is clogged, and at the same time, it is inhibited that the bone grows to the oral cavity side over the implant fixture during the healing stage, there is mainly employed a double technique in which in the oral cavity side of the implant fixture, a cover screw formed such that the end portion thereof in the implant fixture side has a size equal to or larger than that of the end portion of the implant fixture in the oral cavity side, while the oral cavity side thereof has a size larger than that of the end portion in the implant fixture side is screwed in the aforesaid screw hole; a gingiva surrounding the implantation hole is sutured; after not only the implanted implant fixture is thoroughly connected to the mandible of the deficit tooth portion, but also the operated portion where the implantation hole has been formed is healed, the gingiva portion in the oral cavity side having the implant fixture implanted therein is again cut to take apart the cover screw from the implant fixture; in the oral cavity side portion of the implant fixture, a healing abutment in which the outer size thereof in the implant fixture side is equal to that of the end portion of the implant fixture in the oral cavity side, while the length thereof is longer than the thickness of the surrounding gingiva is screwed; and after the cut surrounding gingiva portion has been healed, a fixing device of a dental prosthesis is connected and fixed thereto.
In this double technique, after the surrounding gingiva portion cut in order to screw the healing abutment has been healed; the healing abutment is taken apart from the implant fixture; an abutment having a gold cylinder external engagement portion, which functions as a support for connecting and fixing a fixing device of a dental prosthesis, is connected and fixed to the oral cavity portion of the implant fixture by screwing an abutment screw through which the abutment is penetrated in the aforesaid screw hole; and then, impression taking for preparing a dental prosthesis is carried out. In this case, the gold cylinder external engagement portion provided in the portion of the abutment nearest the oral cavity includes a case that as in posterior tooth portions, it is connected and fixed such that its center line is coincident with the center axis of the implant fixture and a case that as in maxillary anterior tooth portions, its center line is inclined against the center axis of the implant fixture. In any cases, the gold cylinder external engagement portion is connected and fixed so as to enable to fix the position against the implant fixture.
In such dental implant technique, in order to prepare dental prostheses having a good appearance, it is very important to select a construction for each of the abutment having a gold cylinder external engagement portion functioning as a support for connecting and fixing the fixing device of a dental prosthesis and the gold cylinder to be externally engaged in the gold cylinder external engagement portion of the abutment.
That is, in the abutment, a portion in which the end portion thereof in the implant fixture side has a size equal to that of the end portion of the implant fixture in the oral cavity side, while the oral cavity side has a size larger than that of the end portion in the implant fixture side is provided in the implant fixture side, and in the oral cavity side of the portion provided in the implant fixture side, the gold cylinder external engagement portion functioning as a fixing device of a dental prosthesis must be provided. In particular, in order to enable to apply to cases of maxillary anterior tooth portions or cases where an implant fixture as an artificial tooth root portion is not implanted in a mandible of the deficit tooth portion with a right angle, an abutment in which the axial line of the gold cylinder external engagement portion is inclined with an angle against the axial line of the implant fixture has been recently prepared.
In addition, the gold cylinder functioning as the fixing device of a dental prosthesis must be formed such that the end portion thereof in the implant fixture side has the same size as in the end portion in the oral cavity side functioning as a base end portion of the gold cylinder external engagement portion of the abutment. In particular, in case of the gold cylinder as described above, in which the axial line of the gold cylinder external engagement portion is inclined with an angle against the axial line of the implant fixture, it is of a problem how the gold cylinder is externally engaged in the gold cylinder external engagement portion of the abutment.
Hitherto, as the construction for the abutment and the gold cylinder in which the axial line of the gold cylinder functioning as the fixing device of a dental prosthesis is inclined with an angle against the axial line of the implant fixture implanted in the mandible of the deficit tooth portion, there has been proposed a construction in which as disclosed in, for example, JP-A-1-190350, an abutment is provided in the oral cavity side thereof a portion becoming in a cylindrical form having the same diameter after forming such that the end portion thereof in the implant fixture side has the same size as in the end portion of the implant fixture in the oral cavity side, while the oral cavity side thereof has a size larger than that in the end portion in the implant fixture side; in the oral cavity side of the portion becoming in a cylindrical form provided in the oral cavity side, the end surface in the oral cavity side of the expansion portion provided in the state of being further expanded from the aforesaid portion has a plane inclined against the axial line of the implant fixture in the surrounding thereof; in a portion in the center side of the inclined plane and positioned nearest the oral cavity side, a gold cylinder external engagement portion whose center line is inclined against the center axis of the implant fixture is provide; the truncated conical gold cylinder functioning as the fixing device of a dental prosthesis comes into contact with the aforesaid plane in the state that the end surface thereof in the implant fixture side is inclined against the axial line of the implant fixture.
However, in the state that the therapy by means of a dental implant using the abutment and gold cylinder in the above-described conventional example has been completed, as in a anterior tooth portion shown in FIG. 14, an abutment 2* is connected and fixed to an implant fixture 1 by means of an abutment screw 3 while inclining the center line of a gold cylinder external engagement portion 2d* with an angle (against the center axis of the implant fixture 1, and in the oral cavity side of the abutment 2* as well as of the abutment screw 3, a gold cylinder 4* functioning as a fixing device of a dental prosthesis 6 is connected and fixed to the abutment 2* by means of a gold screw 5. In this case, the dental prosthesis 6 is fixed only in the surrounding of the gold cylinder 4*, and if an external surface of an end portion of the dental prosthesis 6 in the side of the implant fixture 1 is larger than an end surface of the abutment 2* in the oral cavity side, bacteria likely retain in this discontinuous surface portion to cause an inflammation in the gingiva. Accordingly, since the dental prosthesis 6 is provided only in the oral cavity side more inside than the end portion of the gold cylinder 4* in the implant fixture 1 side, having the same shape as the end surface of the abutment 2* in the oral cavity side, both the expansion portion of the abutment 2* and the end portion of the gold cylinder 4* in the implant fixture 1 side are exposed, leading to a defect such that the esthetics are remarkably deteriorated.
In addition, as an angled dental implant which is freed from the foregoing defect, there has also been proposed one having a structure comprising a truncated conical portion in which in the above-described conventional abutment, the end portion in the implant fixture side has the same size as the end portion of the implant fixture in the oral cavity side, and the oral cavity side is larger than the end portion in the implant fixture side; and not only in the oral cavity side of this truncated conical portion, an external periphery side plane perpendicular against the axis of the truncated conical portion is provided, but also in a portion positioned in the center side of the truncated conical portion and nearest the oral cavity side, a dental prosthesis retaining portion whose center line is inclined against the center axis of the implant fixture is provided.
However, since this angled dental implant does not use a gold cylinder, a dental prosthesis having such a shape that the end portion in the implant fixture side comes into contact with the above-described external periphery side plane of the abutment and that the dental prosthesis is retained in the dental prosthesis retaining portion of the abutment must be prepared. In addition, since in the portion of this dental prosthesis to be retained in the dental prosthesis retaining portion of the abutment, a bolt hole must be provided in the state that it penetrates from the inclined external surface of the dental prosthesis retaining portion through the center of the truncated conical portion in order to penetrate an abutment screw for connecting and fixing the abutment to the implant fixture, the dental prosthesis must be prepared in such a manner that the presence of this bolt hole does not mind. Accordingly, in order to prepare a dental prosthesis with high precision, this case not only needs many steps as compared with the case where the gold cylinder is used, but also involves a fatal defect that it is difficult to achieve the preparation with high precision.